Fastening or anchoring devices for use in concrete, cement, and similar materials, comprising a helical coil which is expanded into contact with the walls of a hole in the material by the wedging action of a threaded member such as a bolt or screw are well known. Illustrative of such devices are those shown in U.S. Pat. Nos. 806,406, 906,691, 1,199,624, and 3,967,525. In general, such anchors incorporate a helical coil insert which rides on the threads of a threaded member. When driven into a hole of appropriate size, the coil is prevented from rotating by contact with the walls of the hole. Rotation of the bolt expands the coil into locking contact with the walls of the hole, thus providing the desired attachment.
While devices of this type are generally effective for their intended purpose, proper installation thereof may present a problem in determining that the fastener is adequately tightened while at the same time avoiding failure by exceeding the maximum strip-out torque of the anchor. In the devices of the prior art, the torque required to tighten the fastener to its optimum degree of holding power was so close to the torque resulting in strip-out of the assembly, that in many instances the assembly was over-tightened, resulting in failure.